1. Field of the Invention
The invention relates generally to viscosifiable, low thermal conductivity annular fluids and methods of viscosifying, emplacing, and removing the fluids.
2. Background Art
Annular fluids or packer fluids are liquids which are pumped into an annular opening between a casing and a wellbore wall or between adjacent, concentric strings of pipe extending into a wellbore. These fluids are especially necessary in oil or gas well construction operations conducted in low temperature venues of the world, for example, those areas having permafrost. Permafrost is a thick layer of frozen surface ground which may be several hundred feet thick and presents a great obstacle to the removal of relatively warm fluids through a well pipe. Particularly, warm fluid in the well pipe causes thawing of the permafrost in the vicinity of the well resulting in subsidence which can impose compressive and/or tension loads high enough to rupture or collapse the well casing and hence allow the escape of well fluids. In addition, the warm gas or oil coming to the surface in the well pipe becomes cooled by giving up its heat to the permafrost. Further, gas hydrate crystals may form, which can freeze together and block the well pipe. Generally, except for a tiny contribution from radiation, annular heat loss is due to convection and to conduction.
Heavy oil production is another operation which often can benefit from the use of an insulating annular fluid. In heavy oil production, a high-pressure steam or hot water is injected into the well and the oil reservoir to heat the fluids in the reservoir, causing a thermal expansion of the crude oil, an increase in reservoir pressure and a decrease of the oil's viscosity. In this process, damage to the well casing may occur when heat is transferred through the annulus between the well tubing and the casing. The resulting thermal expansion of the casing can break the bond between the casing and the surrounding cement, causing leakage. Accordingly, an insulating medium such as a packer fluid may be used to insulate or to help insulate the well tubing. The packer fluid also reduces heat loss and saves on the energy requirements in steam flooding.
In addition to steam injection processes and operations which require production through a permafrost layer, subsea fields—especially, subsea fields in deep water, 1,500 to more than 6,000 feet deep—require specially designed systems which typically require a packer fluid. For example, a subsea oil reservoir temperature may be between about 120° F. and 250° F., while the temperature of the water through which the oil must be conveyed is often as low as 32° F. to 50° F. Conveying the high temperature oil through such a low temperature environment can result in oil temperature reduction and consequently the separation of the oils into various hydrocarbon fractions and the deposition of paraffins, waxes, asphaltenes, and gas hydrates. The agglomeration of these oil constituents can cause blocking or restriction of the wellbore, resulting in significant reduction or even catastrophic failure of the production operation.
To meet the above-discussed insulating demands, a variety of packer fluids have been developed. For example, U.S. Pat. No. 3,613,792 describes an early method of insulating wellbores. In the U.S. Pat. No. 3,613,792, simple fluids and solids are used as the insulating medium. U.S. Pat. No. 4,258,791 improves on these insulating materials by disclosing an oleaginous liquid such as topped crude oils, gas oils, kerosene, diesel fluids, heavy alkylates, fractions of heavy alkylates and the like in combination with an aqueous phase, lime, and a polymeric material. U.S. Pat. No. 4,528,104 teaches a packer fluid comprised of an oleaginous liquid such as diesel oil, kerosene, fuel oil, lubricating oil fractions, heavy naphtha and the like in combination with an organophillic clay gellant and a clay dispersant such as a polar organic compound and a polyfunctional amino silane. U.S. Pat. No. 4,877,542 teaches a thermal insulator fluid consisting of a heavy mineral oil as the major liquid portion, a light oil as a minor liquid portion, a smectite-type clay, calcium oxide and hydrated amorphous sodium silicate. U.S. Pat. No. 5,290,768 teaches a thixotropic composition containing ethylene glycol and welan gum. The above-discussed patents are herein incorporated by reference.
Although many of the above-described packer fluids function adequately, they fail to meet industrial and governmental concerns for the environment. Particularly, many of the constituents of the above-described packer fluids are unacceptable from an environmental standpoint and are often prohibited for use by government regulation. For example, the mineral oils and heavy crude oils required by several of the above-discussed patents are not permitted for use in areas such as the Gulf of Mexico.
Further attempts at providing insulating annular fluids based on fluids having a more acceptable HSE profile are discussed in G.B. Patent 2,367,315 by Vollmer (hereinafter referred to as “Vollmer '315”). Nothing is taught about insulating annular fluids in U.S. Pat. No. 5,304,620 by Holtmyer, et al. (hereinafter referred to as “Holtmyer '620”), U.S. Pat. No. 5,439,057 by Weaver, et al. (hereinafter referred to as “Weaver '057”), and U.S. Pat. No. 5,996,694 by Dewprashad, et al. (hereinafter referred to as “Dewprashad '694”). However, Holtmyer '620, Weaver '057, and Dewprashad '694 discuss the viscosification of brines using crosslinked hydroxyethylcellulose derivatives. These patents are hereby incorporated by reference.
What is needed, however, are insulating packer fluids that are very low in thermal conductivity while simultaneously meeting all of the other constraints imposed upon the packer fluids and are easier to pump, yet become more viscous than conventional fluids when the insulating packer fluids are resident in situ within the annular space or one of the annular spaces in an oil or gas well.